This invention relates to microwave circuits and, more particularly, to a circuit allowing highly-accurate digital control of signal levels for various system applications.
Digital to analog (D/A) converters are well known for low-frequency applications. Prior art FIG. 1 illustrates a typical D/A low-frequency converter 10 including input buffer 12 and output buffer 20. These buffers 12, 20 can be, for example, operational amplifiers (op amps). A switched resistor ladder network, indicated by dashed lines 15, is coupled between the input buffer 12 and the output buffer 20. The input buffer 12 provides the required input/output impedances and buffering for the switched resistor ladder network 15. The ladder network 15 establishes the digitally-weighted voltages for the D/A converter 10.
The switched resistor ladder network 15 is formed of a series coupling of resistors 14 coupled at one end to the output of op amp 12. At the other end, the series coupling of resistors 14 are coupled through a further resistor 17 to ground. The resistors 14 have a resistive value of "R" and the resistor 17 has a value of "2R". Forming the ladder steps between each of the series coupled resistors 14 is a series circuit of a resistor 16 having a resistive value of "2R" and a switch 18. These steps are coupled at their resistive side between the resistors 14 and at the switch side to an input of the output op amp 20. The number of ladder steps "n" or the converter "resolution" is equal to the number of bits (b.sub.1 -b.sub.n) in the digital control word.
In operation of the circuit of prior art FIG. 1, the input buffer 1 provides high-output impedance and isolation from the voltage source (not shown). The output buffer 20 provides a virtual ground (GND) at the voltage summing node for eliminating impedance dependent summation. The bits b.sub.1 -b.sub.n are selected by closing their respective switches 18. Those bits which are not selected are grounded to maintain the internal impedances of the circuit constant. This type of typical D/A converter using an "R2R" ladder network has a limited frequency range resulting from the parasitic nature of the buffers 12 and 20 and switch devices 18. Because the frequency limit is restricted to operation in the low hundreds of MHz, high-frequency microwave applications are not possible.
Due to the unavailability of a microwave D/A converter, current methods involve using a DC D/A converter followed by buffers and linearizing circuitry which drives a voltage controlled attenuator. In these circuits, the attenuation is proportional to the DC control voltage. Typically however, the proportionality is non-linear. Therefore, lookup tables are provided to store the data for the attenuator versus control voltage. These lookup tables were formed of programmable read-only memories (PROMS) or in other software solutions to aid in the linearization. However, this circuitry is often too slow or requires a very hardware intensive circuit. This consequently increases the cost and labor involved in developing precise control of microwave signals.
There is therefore needed a circuit for allowing highly-accurate digital control of microwave signal levels for various system applications. The circuit should reduce the number of components, size, complexity and power requirements while still providing a low-cost system having improved performance over the prior known devices.
The present invention meets these needs by providing a D/A converter circuit which operates using components available for broad-band microwave operation. The invention provides a digital-to-analog converter circuit for use at RF frequencies, including an input transimpedance buffer receiving an RF input, a complimentary switched R2R ladder network coupled to the input buffer, and an output transimpedance circuit having an extremely low input impedance, coupled to the ladder network and providing an RF output.
It is an advantage of the present invention to provide a microwave multiplying D/A converter using minimal components. This eliminates the need for interface components, such as lookup tables and buffers, previously employed for precise microwave signal control.
It is a further advantage to provide a microwave frequency operating D/A converter offering extreme broadband performance on the order of 12 GHz.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.